NATO foreign ministers suspended civilian and military cooperation with Russia on Tuesday and ordered plans for bolstering defenses in Eastern Europe to show the Kremlin that it will protect allies in the region from any further Russian aggression, alliance sources told news agencies in Brussels.

In their first meeting since Russia occupied and annexed Ukraine's Crimea territory, the top diplomats from the North Atlantic Treaty Organization's 28 member states closed ranks in unanimously voting to increase pressure on Moscow to cease massing troops on Ukraine's border.

An alliance official who briefed reporters in the Belgian capital estimated that the number of Russian troops deployed to Ukraine's eastern boundary at 35,000 to 40,000.

Russian President Vladimir Putin told German Chancellor Angela Merkel in a phone call the previous night that he had ordered a partial withdrawal of Russian forces said to be arrayed along Ukraine's eastern regions for military exercises. Russian media reported the redeployment of a 500-troop contingent but Ukrainian media said that unit was quickly replaced.

NATO chief Anders Fogh Rasmussen said the alliance had yet to see any indication that the Kremlin was attempting to de-escalate the crisis that has ratcheted up East-West tensions to a level not seen since the Cold War.

"Unfortunately, I cannot confirm that Russia is withdrawing its troops," Rasmussen told reporters. "This is not what we are seeing."

A statement issued after the first session of their two-day meeting said the NATO foreign ministers urged Russia "to take immediate steps ... to return to compliance with international law," the Reuters news agency reported. The statement also said NATO would boost cooperation with Ukraine, which is not an alliance member, through joint exercises, training and promotion of military reforms.

Putin has made clear that he considers any NATO alignment with Ukraine to be a threat to his country's traditional sphere of influence. In a demonstration of Kremlin pique at Ukraine's westward leanings, Russia's energy behemoth, Gazprom, on Tuesday announced a 44% increase in the price of its natural gas exports to Ukraine, from $265.50 per 1,000 cubic meters to $385.50.

Gazprom Chief Executive Alexei Miller said the company, in which the Russian government is the majority stakeholder, was canceling a discount previously offered Ukraine due to the country being $1.7 billion in arrears for gas deliveries last year.

The Kremlin in December rewarded then-President Viktor Yanukovich with the discount after he derailed an association agreement with the European Union in favor of closer economic ties with Russia. Yanukovich fled his country in February following months of anti-government protests.

The Russian legislature was also at work Tuesday preparing to repeal another gas discount agreement with Ukraine tied to Moscow's lease of the Crimean port of Sevastopol. Once completed, the repeal will make Russian gas costs for Ukraine about $500 per 1,000 cubic meters, or about 30% more expensive than for Gazprom's European customers.

Russian officials say the 2010 deal is now redundant because Sevastopol and other Crimean bases are no longer Ukraine's territory.

At a background briefing after the foreign ministers' vote to halt programs with Russia, a NATO official was quoted by the Associated Press as saying that the diplomats also ordered planning for possible reinforcement of military assets in Eastern European member states, a possible increase in the alliance "readiness levels" to counter any aggressive action from Russia and a review of NATO's crisis response plans.

Former Soviet republics and allies now in the alliance fear Russia could attempt similar land grabs of their territory using one of the pretexts used to justify the Crimea seizure: the need to protect ethnic Russians or to prevent the Western alliance from incursions into Moscow's historic sphere of influence.

Estonia, Latvia and Lithuania -- former republics of the Soviet Union before its 1991 breakup -- have large populations of Russians and Russian-speakers, and Poland, Romania, Hungary, Bulgaria, Slovakia and the Czech Republic were part of the Moscow-dominated Warsaw Pact during the Soviet era.

Moves were apparently already underway to shore up NATO forces along the eastern flank. Romanian President Traian Basescu said the United States had asked to increase the number of troops and aircraft it has stationed at Mihail Kogalniceanu, north of Constanta, on his nation's Black Sea coast. Washington wants to expand its force from 1,000 to 1,600 and to add military aircraft "for specific missions," Basescu said.

NATO last month increased patrols over the Baltic Sea area and AWACS surveillance of Polish and Romanian airspace after Russia's armed takeover of Crimea and menacing deployments to the border of Ukrainian areas with significant numbers of Russian residents.